Such an adaptive time-discrete filter can be used in a transmission system suitable for full-duplex data transmission or otherwise as a feedback filter in a decision feedback equalizer (DFE), the filter being used for forming a cancelling signal for the "tail" of a received symbol, that is to say the remainder of the signal still received after the actual decision instant of the symbol due to imperfections in the transmission path and capable of disturbing the detection of the symbols coming thereafter, the so-called lagging intersymbol interference. Such a filter can further be used for forming an echo cancelling signal in a transceiver means for full-duplex data transmission over a two-wire circuit.
In two-way digital data transmission over a two-wire circuit, the use of an echo canceller is a necessity for realising an efficient full-duplex data transmission over a transmission channel having a relatively small bandwidth. The imperfections in the coupling of the transceiver means to the two-wire circuit and impedance discontinuities in the two-wire circuit itself result in a signal at the input of the receiver whose major component originates in the transmitted signal from the transmitter associated with the receiver, the echo signal. Reducing the received signal by a signal which is as true an approximation as possible of the trace of the signal transmitted by the associated transmitter, the echo cancelling signal, the actually received signal can be derived from the received signal notwithstanding the strong element of echo comprised therein. In such echo cancellers a transversal filter can be used capable of cancelling any linear combination of a certain number of previously transmitted symbols.
This way of echo cancellation is extensively described in the article by N. A. M. Verhoeckx et al: "Digital Echo Cancelling for Baseband Data Transmission", IEEE Trans. ASSP; Vol. ASSP-27, Nr. 6, December 1979, pp 768-781.
There is a drawback when using transversal filters in that for the echo cancelling of x preceding symbols a filter structure is needed comprising means for obtaining at least x successive versions of the input signal delayed each time over one extra symbol interval and having at least x coefficients. More specifically with echo signals having a long duration, for example an echo "tail" capable of still causing a significant disturbance even after 100 symbol intervals, this will lead to very complex and especially costly filter structures, among others due to the required memory capacity for all filter coefficients.
In duplex transmission over a two-wire circuit, the transceiver means is generally coupled to this circuit via a circuit comprising a transformer. This transformer is advisable for protecting the transceiver means from any high voltage peaks on the two-wire circuit and for converting the unbalanced transmitted signal into a balanced signal. Besides, the authorities controlling a two-wire transmission network generally demand the use of such a transformer for obtaining a balanced termination of the two-wire circuit.
As a result of the use of a transformer between the transceiver means and the two-wire circuit, the echo signal resulting from the imperfections in the coupling circuit can be regarded as a transmitted signal reaching the receive part via a channel having a frequency dependent response curve. More specifically, in data signals having a high energy content in the low-frequency portion of the spectrum this can lead to echo signals with an extremely long "tail", which means echo signals capable of causing a significant disturbance of the signal transferred via the two-wire circuit even after 100 symbol intervals. As elucidated hereinbefore, it is unattractive to cancel such echo signals of a duration of many dozens of sumbol intervals by means of a transversal filter.
Needless to observe that a line code can be used for the data signals having smaller energy contents in the low-frequency portion of the spectrum. The duration of the echo signal can thus be restricted. However, the adverse effect of such line codes for data signals will be a larger energy content in the high-frequency portion of the spectrum and consequently higher attenuation on transmission, which will restrict the applicability of such signals.
The duration of the echo-signal can also be restricted by means of linear equalizing techniques in the receive part. However, in a digital implementation of the transceiver means, digital word multiplications will have to be used resulting in a complex and consequently costly circuit.
Conversely, the use of a more refined two-wire-four-wire coupling (hybrid junction) will also achieve a shortening of the unintentionally long duration of the echo component. However, this solution will largely entail falling back on analog techniques, which is undesired, as in the currently used data transmission systems a complete integration of digital techniques in the transceiver arrangements is strived for.
There are filters which are able to produce with a limited number of coefficients a cancelling signal stretching out over a much longer period than the cancelling signal obtainable from a transversal filter with an identical number of coefficients. Such filters are known as recursive filters. If the filter coefficients of these filters are adjusted adaptively, however, the problem will arise that the stability of the filters is not guaranteed under all circumstances, which problem is not known when using transversal filters, and a further disadvantage of these recursive filters is that the required mechanism for adaptively adjusting the filter coefficients is harder to implement than with a transversal filter.
The article entitled "Untersuchungen und Entwurfe von hochintegrierbaren Echokompensationsverfahren zur Duplexubertragung" by S. Hentschke, published in Frequenz, Vol. 36, No. 11, pp 302-309, November 1982, has disclosed how a complete cancelling of an echo signal having a long duration can be achieved by combining a transversal filter with a recursive filter. It was suggested in the aforementioned article to cancel the echo signal of the most recently transmitted symbols, for example the last 32 symbols,, by means of an adaptive transversal filter and to cancel the echo signal coming from the send symbols, transmitted more than 32 symbol intervals before that by an adaptive recursive filter, the latter filter for example capable of containing four adaptive filter coefficients.
For this purpose the transmission signal is applied to an adaptive transversal filter having N=32 coefficients and the signal is likewise applied to the input of an adaptive recursive filter having four coefficients after attenuation over N=32 send symbol intervals. The echo cancelling signal is obtained by summing the output signal of the transversal filter and that of the recursive filter. The filter coefficients of the two filters are adjusted in a known way by means of an adaptive control loop deriving its control signal from the transmitted signal and the received signal, after the latter has been reduced by the echo cancelling signal.
A drawback of the known solution is that considerable problems will be encountered with respect to the stability of the echo cancelling filter, that the solution of two separate filter structures, each having a number of adaptively adjustable coefficients will require much storage capacity and consequently be very costly and that the found solution will needlessly be complicated.
Thereto the invention has for its object to find a solution for the abovedescribed problem which solution is neither complicated nor costly and, which does not know any problems of stability and nevertheless enables an optimum cancelling signal to be produced even with signals to be cancelled having a duration of many dozens of send symbol intervals.